1. Field of the Invention
The invention relates to a communications apparatus, and more particularly to a communications apparatus capable of simultaneously providing more than one radio access technology (RAT) communications standard with transmission noise suppression.
2. Description of the Related Art
With advancements in communications techniques, mobile stations (MS, which may be interchangeably referred to as user equipment (UE)) are now capable of handling multiple radio access technologies (RAT), such as at least two of GSM/GPRS/EDGE (Global System for Mobile Communications/General Packet Radio Service/Enhanced Data rates for Global Evolution), W-CDMA (Wideband Code Division Multiple Access) RAT, WiFi (Wireless Fidelity), LTE (Long Term Evolution), or the like via one communications apparatus.
Generally, different RATs operate in different frequency bands. However, some of them may still operate in a frequency band that is close to or even overlaps with the operating band of one or more RATs. For example, the LTE band 40 (2300 MHz-2400 MHz) is very close to the ISM (Institute for Supply Management) band (2400 MHz-2483.5 MHz), and there is nearly no guard band between the LTE band 40 and the ISM band.
When considering the non-linearity of radio-frequency (RF) devices utilized in a radio module, high-order inter-modulation (IM) terms may be generated and occupy a wide range of frequency bands. For example, a power amplifier may generally generate the high-order IM terms and wideband noise. Therefore, when two radio modules having operating bands that are close to or overlap each other are integrated into one communications apparatus, mutual interference may occur when one is transmitting uplink signals and the other one is receiving downlink signals, since the transmitted uplink signals may leak to (that is, be captured by) the antenna of the receiving radio module. Those IM terms and wideband transmitter noise are together called TX skirts.
FIG. 1 shows a frequency spectrum of the WiFi transmitting signal and LTE receiving signal. As shown in FIG. 1, the TX skirts (TX_Skirt) of the WiFi transmitting signal (WiFi_TX_Signal) leaks to LTE operating bands and becomes noise that interferes with the LTE receiving signal (LTE_RX_Signal). This is even worse when the transmission noise is so strong because the two radio modules are usually disposed very close to each other when integrated into one communications apparatus.
Therefore, a communications apparatus capable of simultaneously providing more than one radio access technology (RAT) communications standard with transmission noise suppression is required.